Existing signalling methods used for the return of information from a downhole sensor, for example in the oil and gas extraction industries, rely on the transmission of signals along suitable conductors. These conductors must traverse the entire depth of the hole in which the transducer is located. There is significant cost associated with installing a downhole conductor, and therefore it is desirable to multiplex the downhole sensor signals along other conductors that are also present in the borehole.
It has previously been proposed to employ the three-phase conductors used for electrical supply to downhole motors that may be located in the wellbore. It is a characteristic of a balanced three-phase electrical supply that after passing through the load (such as a motor), the conductors can then be grounded at a neutral point. The neutral point will, in the absence of faults in the three-phase supply, exist at substantially zero volt, with respect to the surrounding earth or chassis potential. A mirror of the neutral point can then be constructed above ground through the use of three inductive loads, and in this way a conductive path can be created between the neutral point on the surface and of the downhole neutral point with the return signal going through the downhole casing or tubing in the wellbore. Signals can be sent around this conductive path.
However, if a fault should occur in the three-phase supply then the downhole neutral point would be exposed to significant voltages. Accordingly, a downhole inductor is commonly placed in series with the neutral point prior to connection to the signalling instrumentation. In association with a downhole capacitor, a low pass filter is created that limits the ingress of the high alternating voltages that power the downhole motor. This inductor and capacitor combination will tend to limit the speed at which the applied signal can be varied and hence the bandwidth of this transmission path. Typically, given the voltages involved in this context, an inductance will be required which will impose a minimum settle-time of the order of one second. That is, after an abrupt change in the applied voltage at the downhole neutral point, there will be a period of approximately one second during which the voltage at the surface neutral point will be unstable before it settles at the new voltage level. During this time the measured voltage at the surface neutral point will be highly variable as a result of the inductor.
Known signalling systems such as U.S. Pat. No. 5,539,375 use voltage or current levels to encode information, but these methods suffer from this long settling time caused by the above mentioned inductors in the signal path. In addition, electrical interference from the pump power can cause the current or voltage levels to be disturbed, degrading the signal quality if analogue encoding methods are used, or degrading the data transmission rate if digital encoding methods are used.